Center-pole magnetic circuit

ABSTRACT

A field-producing permanent magnet fitting within the annular cathode of a crossed-field device, such as the magnetron. The magnet is an annular device having an inner cylinder of highpermeability steel forming a flux return path for the middle cylinder which is made of radially gaussed samarium cobalt material, and an outer section comprising two rings of highpermeability steel, one at each end of said middle section, forming pole pieces for the distribution of flux thru the interaction region of said crossed-field device.

United States Patent 1191 MacMaster et al.

1451 Dec. 17,1974

CENTER-POLE MAGNETIC CIRCUIT 3,289,036 ll/l966 Downing et al 315/3975 X 3,325,758 6/1967 Cook 335/306 X [751 mentors: Gemge MacMaser Lexmgton; 3,346,766 10/1967 Feinstein.... 315/3971 Kenneth Dudley, Sudbury, both 3,376,466 4/1968 Gerard 315/3971 of Ma5$- 3,755,706 8/1973 Scott 335/296 X [73] Assignee: The United States of America as I represented by the Secretary of the Przmary Exammer-James W. Lawrence Navy Washington DC Assistant Examiner--Saxfield Chatmon, Jr. Attorney, Agent, or Firm-R. S. Sciascia', P. Schneider [22] Filed: Nov. 1, 1973 [21] Appl. No.: 411,617 [57] ABSTRACT A field-producing permanent magnet fitting within the [52] s C] 315 3 71 313 157 315 395 annular cathode of a crossed-field device, such as the 335 229 335/302 335 30 magnetron. The magnet is an annular device having an 51 Int. Cl. 1-10'1 25/50 inner cylinder of high-permeability steel forming a flux [58] Field of Search 315/3951, 39.71; 313/157; return path for the middle cylinder which is made of 33 5 229 232 .3 3 29 radially gaussed samarium cobalt material, and an outer section comprising two rings of high-permeabil- 5 References Cited j ity steel, one at each end of said middle section, form- UNITED STATES PATENTS ing pole pieces for the distribution of flux thru the in- 2 235 517 3,194! E 1 313/157 X teraction region of said crossed-field device.

spe 1 2,411,953 12/1946 Bmwn..;l 315/3971 X 10 Claims, 4 Drawing Figures TUBE AXIS I |4 was 2 MIDPLANE se -i 9'12 32 CENTER-POLE MAGNETIC CIRCUIT BACKGROUND OF THE INVENTION This invention relates to crossed-field devices (CFD) and especially to a center-pole magnet for establishing the magnetic field in a CFD.

Certain types of electronic tubes use a magnetic field transverse to the electron discharge path between the cathode and the anode. These tubes may be termed crossed-field devices, and the magnetron is an example of this type of electron tube. V

In a CFD it is desired that the magnetic field in the interaction space (between the cathode and the anode) be as uniform and as nearly parallel to the cathode surface as possible. Most CFDs utilize large, heavy Alnico magnets outside the tube vacuum envelope to provide the magnetic field and the weight and size of the magnets make the tubes very cumbersome.

SUMMARY. OF THE INVENTION The present invention comprises a magnet with an annular inner pole section encircled by an annular permanent magnet. Thesesections are roughly cylindrical in shape and fit inside an annular cathode in a CFD. An annular pole piece is set on each end of the permanent magnet to overhang and direct flux through the interaction space between the cathode and anode of the CFD.

An object of the invention is to reduce the size of the magnet'or magnets used with current crossed-field devices.

Another object is to provide an magnet for a CFD small enough to be located inside the cathode structure.

A further object is to increase the Gauss-carrying capacity of an internal magnet in a CFD by providing a high-permeability center pole for the magnet.

Yet another object is to'providea new magnetic circuit for a CFD which can produce a high-density, annular. field with very little magnetic material.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational cross-sectional view of a magnet according to this invention and its physical location with respect to the cathode and anode of a CFD;

FIG. 2 is a top view of the magnet showing its annular structure; I

FIG. 3 is a plot showing the magnetic field of a center-pole magnet; and

FIG. 4 is an elevational cross-section of another magnet with a thinner center pole.

DETAILED DESCRIPTION FIG. 1 shows a magnet in accordance with the invention in a CFD tube only the cathode and anode of which are shown. The magnet, shown generally by numeral 10, fits inside a cylindrical cathode 12. Spaced at a short distance away is an annular anode 14. The axis 16 of the tube is a vertical line which passes through the middle of the magnet 10; the axis of the magnet is identical with the tube axis.

. ercive force or resistance to demagnetization and have energy products as high as 20 X 10 gauss-oersteds. In a microwave tube, samarium cobalt can replace several times its weight in Alnico. The central portion of the middle section 18 is tapered 24 to balance out leakage flux. When the leakage is exactly balanced out, an axial field with excellent uniformity can be obtained. By varying the tapered section, the field shape can be made straight, barreling or antibarreling. The middle section (permanent magnet section) is tapered 30 at its ends to lengthen the leakage path without using much magnetic material.

The inner section 20, or center pole, is substantially a cylindrically shaped section with a hollow center 26 thru which the magnet and tube axes pass. The cylinder is tapered 28 atboth ends to make it lighter. The center pole 20 is made of a high-permeability material, such as steel. This flux return path down the center of the magnet 10 results in reduced leakage, less iron in the magnet, and a shorter return path, thus requiring much less samarium cobalt to obtain the required field strength.

The outer section 22 is actually formed in two portions, an upper ring and a lower ring, with an outer flange 34 facing downwards on the upper ring and upwards on the lower ring. These rings 22 and 22 constitute field-forming poles, or field-forming pole end shields. The flanges direct the magnetic field into the interaction region 36 between the cathode 12 and the anode 14 in a direction substantially parallel to the surface of the cathode. a

The tube midplane 32 passes through the centers of the anode, the cathode and the magnet 10 and will be called the transverse axis of the magnet.

FIG. 3 shows a plot of the magnetic field flux lines 21 through the magnet 10. Note the\substantially uniform field in the interaction region 36 and the concentration of flux lines in the center pole 20.

FIG. 2 shows a top view'of the magnet 10, indicating the annular shapes of its component parts. v

FIG. 4 shows another magnet with a thinner center pole 20 to reduce the weight of the magnet. The tube structure is shown in a little more detail. The magnet, cathode and anode are shown held together by a frame 40, a high-voltage spacer 38 being placed between the frame which is at cathode potential and the anode.

In a magnetron, this magnetic circuit design can supply greater than 2,000' gauss in a 0.5 inch gap at 1.0 inch diameter with less than 5 percent variation. The samarium cobalt magnet weighs less than 3 ounces and the whole magnetic circuit less than 6 ounces. A conventional Alnico magnetic circuit to supply this amount of field with this uniformity would weigh several pounds. If the latter weight were 3 pounds, this would amount to a reduction in weight of approximately percent.

The new center-pole magnetic circuit produces the desired field and field geometry with very little samarium cobalt magnetic material. This design is less restrictive of cathode size, produces a more uniform field geometry and a stronger field than previous CFD magnetic circuits.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A magnet comprising:

a middle section formed in a cylindrical shape with an inwardly tapered central portion, said middle section being formed from a material which is permanently magnetized and is radially gaussed;

an inner section formed in a cylindrical shape and placed radially inward of and abutting said middle section, said inner section being fabricated from a high-magnetic-permeability material and forming a flux return path; and

an outer section comprising two rings fabricated from a high-magnetic-permeability material, each ring comprising a pole piece, said rings being placed abutting the outer surface of said middle section, one ring near the top thereof and one near the bottom, so that said pole pieces face each other and a substantially uniform magnetic field extends therebetween.

Z A magnet as in claim 1, wherein each said ring is formed with a peripheral flange extending axially downward therefrom, and forming said pole piece, said rings being disposed so that said flanges face each other.

3. A magnet as in claim 1, wherein said middle section is formed from samarium cobalt.

4. A magnet as in claim 1, wherein said inner section is tapered inwardly at both ends.

5. A magnet as in claim 1, wherein said inner and outer sections are fabricated from steel.

6. In a crossed field device having an annular cathode and an annular anode structure spaced radially outward therefrom, a magnet comprising:

a middle section formed in a cylindrical shape with an inwardly tapered central portion, said middle section being formed from a material which is permanently magnetized and is radially gaussed;

an inner section formed in a cylindrical shape and placed radially inward of and abutting said middle section, said inner section being fabricated from a high-magnetic-permeability circuit material and forming a flux return path; and

an outer section comprising two rings fabricated from a high-magnetic-permeability material, each ring comprising a pole piece, said rings being placed abutting the outer surface of said middle section, one ring near the top thereof and one near the bottom, so that said pole pieces face each other and a substantially uniform magnetic field extends therebetween.

7. A magnet as in claim 6, wherein each said ring is formed with a peripheral flange extending axially downward therefrom and forming said pole piece, said rings being disposed so that said flanges face each other.

8. A magnet as in claim 6, wherein said middle section is formed from samarium cobalt.

9. A magnet as in claim 6, wherein said inner section is tapered inwardly at both ends.

10. A magnet as in claim 6, wherein said inner and outer sections are fabricated from steel. 

1. A magnet comprising: a middle section formed in a cylindrical shape with an inwardly tapered central portion, said middle section being formed from a material which is permanently magnetized and is radially gaussed; an inner section formed in a cylindrical shape and placed radially inward of and abutting said middle section, said inner section being fabricated from a high-magnetic-permeability material and forming a flux return path; and an outer section comprising two rings fabricated from a highmagnetic-permeability material, each ring comprising a pole piece, said rings being placed abutting the outer surface of said middle section, one ring near the top thereof and one near the bottom, so that said pole pieces face each other and a substantially uniform magnetic field extends therebetween.
 2. A magnet as in claim 1, wherein each said ring is formed with a peripheral flange extending axially downward therefrom, and forming said pole piece, said rings being disposed so that said flanges face each other.
 3. A magnet as in claim 1, wherein said middle section is formed from samarium cobalt.
 4. A magnet as in claim 1, wherein said inner section is tapered inwardly at both ends.
 5. A magnet as in claim 1, wherein said inner and outer sections are fabricated from steel.
 6. In a crossed field device having an annular cathode and an annular anode structure spaced radially outward therefrom, a magnet comprising: a middle section formed in a cylindrical shape with an inwardly tapered central portion, said middle section being formed from a material which is permanently magnetized and is radially gaussed; an inner section formed in a cylindrical shape and placed radially inward of and abutting said middle section, said inner section being fabricated from a high-magnetic-permeability circuit material and forming a flux return path; and an outer section comprising two rings fabricated from a high-magnetic-permeability material, each ring comprising a pole piece, said rings being placed abutting the outer surface of said middle section, one ring near the top thereof and one near the bottom, so that said pole pieces face each other and a substantially uniform magnetic field extends therebetween.
 7. A magnet as in claim 6, wherein each said ring is formed with a peripheral flange extending axially downward therefrom and forming said pole piece, said rings being disposed so that said flanges face each other.
 8. A magnet as in claim 6, wherein said middle section is formed from samarium cobalt.
 9. A magnet as in claim 6, wherein said inner section is tapered inwardly at both ends.
 10. A magnet as in claim 6, wherein said inner and outer sections are fabricated from steel. 